1. Field of the invention
The present invention relates to an injection apparatus for melted metals used for injection molding nonferrous metals having a low melting point, such as zinc, magnesium, or alloys thereof, completely melted in liquid phase.
2. Detailed Description of the Prior Art
Attempts have been made to completely melt nonferrous metals having a low melting point so as to allow injection molding in liquid phase. Like in the case of injection molding of plastics, the molding method adopts a heating cylinder having inside an injecting screw, which is allowed to rotate and move along the axial direction. Granular metals supplied from the rear portion of the heating cylinder are heated and melted completely by shear heat and external heat while being transferred toward the fore end of the heating cylinder by means of rotation of the screw. After a quantity of the melted metals in liquid phase is metered in the fore portion of the heating cylinder, the metals are injected into a mold through the nozzle attached to the tip end of the heating cylinder by the forward movement of the screw.
Problems occurring in case of adopting the foregoing injection molding for the metals are, for example, difficulty on the transfer of the material by means of rotation of the screw, the maintenance of the temperature of the melted metals in liquid phase, unstable metering, or the like.
A melted plastic material has a high viscosity, and transfer of the melted plastic material by means of rotation of the screw is allowed mainly because a friction coefficient at the interface of the melted plastic material and the screw is smaller than a friction coefficient at the interface of the melted plastic material and the inner wall of the heating cylinder, and therefore, a difference in friction coefficient is produced between the two interfaces.
In contrast, the metal completely melted in liquid phase has such a low viscosity compared with the plastic material that a difference in friction coefficient is hardly produced between the above two interfaces. Hence, a transfer force such as the one produced with the melted plastic material by means of rotation of the screw is not readily produced.
However, a transfer force is produced with the metals in solid state and in a high viscous region where the metals are in a semi-molten (liquid-solid) state during the melting process. Thus, the metals can be transferred by means of rotation of the screw up to that region. Nevertheless, as the metals are further melted, the viscosity thereof drops with an increasing ratio of the liquid phase, and the transfer force produced by the screw grooves between the adjacent screw flights decreases, thereby making it difficult to supply the melted metals in a stable manner to the fore end portion of the heating cylinder by means of rotation of the screw.
Because the melted plastic material has a high viscosity, it is stored in the fore end of the heating cylinder by means of rotation of the screw, while at the same time, a material pressure pushing the screw backward is produced as a reaction. By controlling the screw retraction caused by the material pressure, a constant quantity of the melted material can be metered each time.
However, the metals in the low-viscous liquid phase cannot produce a pressure high enough to push the screw backward. Thus, the screw retraction by the material pressure hardly occurs, and if the metals are reserved in the fore end portion by means of rotation of the screw alone, a quantity thereof undesirably varies, thereby making it impossible to meter a constant quantity each time.
In addition, the metals have a far larger specific gravity compared with the plastics, and have a low viscosity and fluidity in liquid phase. For this reason, when allowed to stand by stopping rotation of the screw, the metals in liquid phase in the heating cylinder placed in a horizontal position leak into the semi-molten (liquid-solid) region in the rear portion through a clearance formed between the screw flights and the heating cylinder. Consequently, the metal material metered in the fore end portion causes a back flow onto the periphery of the fore portion of the screw through the opened ring valve, and the quantity thereof is undesirably reduced.
The liquid level in the fore end portion is lowered with the decreasing reserved quantity. For this reason, a gaseous phase (space) that makes the metering unstable is generated at the upper portion of the fore end portion. In addition, the leaked liquid phase material increases its viscosity in the semi-molten (liquid-solid) region as its temperature drops, or turns into solid depending on the heating condition in the semi-molten (liquid-solid) region, thereby forming weirs in the screw grooves. This poses a problem that the granular material supplied from the feeding opening provided behind the weir cannot be transferred readily by means of rotation of the screw.
The present invention is designed to solve the problems stated above in the injection molding of the metals in liquid phase. An object of the present invention is to provide a new injection apparatus which can easily and smoothly transfer the metals, melt them by the external heat, meter and degas by employing a reservoir to reserve metals in liquid phase for the injection screw, and a method for injection molding.
In order to achieve the above-mentioned object, the present invention according to the first aspect provides an injection apparatus for melted metals, comprising a heating cylinder having a fore end portion which communicates with a nozzle member and of which internal diameter is made smaller to serve as a metering chamber having a required length, and an injection screw installed within the heating cylinder to be movable and rotational, a tip end of the injection screw being formed in a plunger having a diameter which is almost the same as that of the metering chamber and can insert into the metering chamber while keeping a clearance for sliding, wherein a reservoir consisting of an axial portion is provided between the plunger and a feeding portion containing screw flight around the axial portion.
Moreover, the present invention provides the injection apparatus for melted metals according to the foregoing aspect, wherein a projected portion for limiting the feeding of granular metals flowing from the feeding portion to the reservoir with metals in liquid phase and for preventing the metals in liquid phase reserved in the reservoir from flowing backward when the injection screw moves forward is provided on a boundary between said feeding portion and the reservoir.
The present invention further provides the injection apparatus for melted metals according to either of the foregoing aspects, wherein the screw flight of the feeding portion is provided in such a manner that screw groove of the screw end is placed immediately below the feeding opening at the rearmost position of the screw in the heating cylinder, and that the screw end is placed in front of the feeding opening at the foremost position of the screw to close the feeding opening with the axial rear portion of the screw portion without screw flights, and to be capable of achieving transferring of the granular metals by the screw rotation at the rearmost position of the screw.
The present invention further provides the injection apparatus for melted metals according to the foregoing aspects, wherein the screw flight of the feeding portion is provided in such a manner that a screw groove of a screw end is placed immediately below the feeding opening at the foremost position of the screw in the heating cylinder, and that the screw end is placed behind the feeding opening at the rearmost position of the screw to be capable of achieving transferring of the granular metals by the screw rotation at the foremost position of the screw.
Moreover, the present invention provides the injection apparatus for melted metals according to the first aspect, wherein the plunger is provided with a heat-resistant seal ring therearound, and a flow-through hole is formed therein from a ring groove for fitting the seal ring to a conical end of the plunger.
The present invention further provides the injection apparatus for melted metals according to any of the foregoing aspects, wherein the heating cylinder is installed with an inclination and positioning the feeding opening higher than the nozzle to allow the metals in liquid phase to flow down into the reservoir by its own weight.
In the construction stated above, a reservoir for the metals in liquid phase is provided between the plunger as a fore end portion and a feeding portion. By means of retracting the injection screw, the metal temporarily reserved in the reservoir is allowed to be reserved in the above-mentioned metering chamber. Thereby, the next feed of metals is completely melted and the temperature thereof is maintained while they are maintained in the reservoir even if the metals are melted by the external heat. As a result, the temperature of metals can be kept constant.
Since a compressing portion to generate shear heat is unnecessary, the depth of the screw grooves between the screw flights can be made constant so as to feed the metals smoothly. Thereby the metals evenly contact the inner surface of the heating cylinder so that a fluctuation of temperature rarely happens. Since the most part of the metals melt into liquid phase while they reach to the projected portion on the boundary to the reservoir, and large granules which are incompletely melted are prevented from flowing into the reservoir by means of the projected portion, the metals in the reservoir are melted completely into the liquid phase and always ensured that they will be reserved into the metering chamber.
Furthermore, in the construction stated above, while the screw moves forward and the feeding opening is being closed with the axis, the feeding of the metals will be automatically limited upon the start of injection. It prevents congestion of the metals in the screw grooves in the rear of the screw. Thereby, a friction by rotation and sliding to the screw is decreased, which stabilizes melting and injecting of the metals to improve the quality of molded products.
The heating cylinder is inclined downward so as to reserve the melted metals in the reserving space surrounding the axial portion in the front portion of the heating cylinder. Therefore, even if the metals are in the liquid phase of a low viscosity, they will not flow backward so that the reserved amount will not fluctuate. In addition to it, since the rotation of the screw supplies the metals in liquid phase, in spite of injection molding the metals in liquid phase, a stable quality of molded metal products can be produced.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.